Modelling of particulate matter transformations and capture efficiency

Conference poster, 2013

Open filters (with low pressure drop) have potential for energy efficient reduction of particulate matter (PM) from internal combustion engines and for some applications, open substrates may even suffice. However, the PM from internal combustion engines is prone to changes (via changes in gas composition and temperature) and it is therefore very challenging to characterize. The modelling of these systems is also challenging due to the variability in size-dependent heterogeneous properties. The capture efficiency of PM in open substrates (bare cordierite and alumina-coated cordierite monoliths) has been investigated using PM from a real engine under various flow conditions (varying residence times and temperatures) and sampling settings (dilution ratios) using a DMS500 from Cambustion [1]. However, the capture efficiency was affected by removal of volatiles (hydrocarbons) influencing both size and numbers. In order to quantify these effects, a conceptual model has been implemented that can be used as an in-situ analyser of the PM properties. By applying sigmoidal functions to describe the distribution between pure solids (soot) and semi volatiles (soot cores with HC outer shell), an empirical but physically sound model was achieved. The number of parameters was relatively low (8 parameters) but experimental data was not very rich and a significant parameter correlation was observed. The results show how exhaust treatment (heating and/or dilution) changes the characteristics of the PM and how the model could be used to describe the experiments.. In the current work, modelling issues are described. The capture efficiency is modelled as one single channel, discretized using tanks-in-series. In each tank, evaporation of hydrocarbons makes the particles shrink, which affects the diffusivity and hence the capture to the walls. Furthermore, the methodology for parameter estimation is elaborated, and a user-defined Jacobian is applied to make the optimisation problem more stable. This situation (with many correlated parameters) is also common for heterogeneous catalysis modelling, which makes the methodology interesting also for a wider audience. The obtained HC characteristic is also implemented in CFD simulations where more realistic flow conditions are investigated and the impact of the fast evaporation process is assessed. In the development of more detailed and accurate chemical kinetics for the reactions of diesel and gasoline particulate matter in filters, it will be necessary to also be able to predict the time-resolved properties of the particles collected in the filter (i.e. reactivity, amount of adsorbed hydrocarbons, etc.). The important implication of this work is therefore that such models can be constructed with the aid of carefully gathered data from experiments on open substrates.